GaAs is a compound which is commonly used in semiconductor wafers. The material may either be used on the frontside or the backside of the semiconductor wafer. When GaAs is used on the wafer frontside, it is typically necessary to decrease the GaAs thickness by small amounts, for example, less than one micron, to manufacture the integrated circuit. Whereas, when GaAs is used on the backside of a wafer it is often necessary to decrease the GaAs thickness by a much greater amount, for example, up to 500 microns. Therefore, different processing steps have been developed for thinning GaAs layers, depending on whether frontside or backside thinning is desired.
One process which is commonly used to thin GaAs on the backside of a semiconductor wafer is known as a back lap process. This technique involves the use of wax to mount the wafer, backside up, on a carrier which is then mounted on a block. Wet alumina is deposited between the wafer backside and a lapping wheel. The wheel is then rotated so that the GaAs is gradually removed by physically grinding the wet alumina against the wafer backside. After most of the desired thinning has been achieved, the wafer surface must be polished by grinding again with a finer grain.
There are several significant drawbacks with the back lap thinning process. First, it is very difficult to uniformly thin the GaAs layer with this method. The wafer mounting steps on the carrier and the block are extremely critical. If the wafer is not mounted perfectly even, the lapping wheel will not uniformly lap the GaAs layer. Second, the back lap thinning process is very time consuming. For a 3" wafer it can take up to approximately three hours to thin 500 microns off of a GaAs layer. Third, the back lap procedure is a dirty process which must be performed outside the clean room environment.
Another commonly used GaAs thinning process involves immersing the wafer in an etchant solution. In this approach a wafer is mounted on a carrier, backside up. The carrier and wafer are usually then suspended in an etchant solution contained in a vessel such as a beaker. During the etching process the beaker is rotated and tilted to cause uniform circulation of the etchant around the wafer. The amount of GaAs thinning is supposedly controlled by controlling the immersion time. One notable problem with the immersion approach is that in order to achieve the GaAs thinning in the desired time frame, highly toxic etchant materials, such as hydrofluoric acid, nitric acid or acetic acid, are typically used. Another problem with this approach is that it is difficult to achieve the desired thickness uniformity in actual production or when processing large wafers.